Known as functional alloys exhibiting superelasticity are Au--Cd alloys, Cu--Zn--Al alloys, Cu--Al--Ni alloys, Ni--Ti alloys and the like.
Of those functional alloys, Ni--Ti alloys have attracted attention, for example, as orthopedic implant materials due to their superiority in corrosion resistance. However, Ni--Ti alloys, in contact with body tissue, are known to cause inflammation to the contact part. Moreover, the effect of dissolved Ni ions on body tissue (e.g., carcinogenesis) has not been fully studied and safety has not yet been confirmed. Therefore, Ni--Ti alloys cannot be implanted into the body as they are (i.e., without coating). Furthermore, since nickel in Ni-containing articles such as pierced earrings, wristwatch bands or the like, which are used in contact with the skin, dissolves in sweat and induces allergies, it might be highly dangerous to use Ni--Ti alloys as implant materials.
Known as Ni-free shape memory alloys are Ti alloys (U.S. Pat. No. 4,412,872), etc. However, there are no known Ni-free superelastic alloys that are easy to use as structural materials for industrial use and have high corrosion resistance and improved workability.